1. Field of the Invention
The invention pertains to the field of aircraft flight control and more specifically to the automatic control of the vertical axis trajectory during a path capture maneuver.
2. Description of the Prior Art
Generally the performance of a vertical axis path capture entails maintaining or establishing an acceptable closure rate which is within aircraft performance limits, accommodating late system engagement by the flight crew, limiting all maneuvers to accelerations within the passenger comfort range, and designing a closed loop control system which is stable with minimum overshoot.
In a Flight Management System (FMS) the problem is aggravated by the requirement to track a curved path approximated by a series of short, straight segments. During a capture maneuver, it is actually required that the system "look ahead" to determine if it should be attempting to capture a downstream path segment.
Predominantly, the prior art solved the problem by implementing three control modes and associated control mode switching logic; a vertical speed mode for those special instances where establishing an acceptable path closure rate is required, a path capture mode which executes the capture maneuver, and a path hold mode for controlling the airplane on the straight path. In some implementations, a non-linear path capture control law was used to cause a constant acceleration capture rather than an exponential capture characteristic of linear controllers. This concept satisfied the need to control normal acceleration during capture maneuvers but created stability problems within the closed loop. Control mode switching logic was complex in order to handle all the special cases, such as insufficient path closure rate and late pilot engagement. Control mode switching logic is especially complex in a FMS.
Additionally, the prior art systems provided marginal control stability of the nonlinear control law used in the constant acceleration capture maneuver. The capture maneuver was designed with a fixed acceleration command which made variable "g" captures impossible. Multiple control laws required excessively complicated logic to select the controllers as well as to initialize them. With the lack of variable "g" captures along with the inability to "look ahead" during multiple segment tracking, airplane performance was substandard under all but the most nominal conditions.
One prior art concept applies a forcing function generator to the autopilot for capturing altitudes. This is implemented with a linear, second order filter which adjusts the natural frequency and damping factor to control capture performance. However, no constant acceleration command capability, built-in acceleration or position limiting is provided.